1. Field of the Invention
The present invention relates to a multi-phase claw-pole type rotary machine and, more particularly, relates to a two-phase or three-phase permanent magnet (PM) type stepping motor reduced in cogging torque.
2. Description of the Prior Art
FIG. 1A and FIG. 1B show a two-phase PM type stepping motor comprising first and second rotary machine elements each for single phase arranged coaxially with each other. Each of the first and second rotary machine elements comprises an inner magnet rotor magnetized in a circumferential direction thereof so as to form N pole and S pole alternately, a claw-pole type outer stator arranged concentrically with the inner magnet rotor with an air gap therebetween, and an annular stator winding. The claw poles of the stator are formed on one side and the other side of the stator as shown in FIG. 1B. The claw-poles of each side are the same in number with magnetic poles of the rotor, and a magnetic flux entered into the claw pole of one side is returned to the claw pole of the other side passing through the annular winding. In case of a two-phase motor, two single-phase motors are connected coaxially with a phase difference of 90° (electric angle). In case of a three-phase motor, three single-phase motors are connected coaxially with a phase difference of 120° (electric angle).
In the conventional claw-pole type motor, the pitch of the claw poles of the stator is equal to the pitch of the magnetic poles of the rotor, so that the cogging torque becomes large.
A PM type stepping motor reduced in cogging torque by varying the pitch of the claw poles of the stator is disclosed in the Japanese Patent Application Laid-Open No. 136631/1998 and the Japanese Patent Application Laid-Open No. 161055/2001.
However, in such conventional stepping motor, the magnetic attraction force becomes unbalanced because the interval of the claw poles is varied so as to form a too narrow portion unreasonably or the position of the claw poles becomes asymmetric with respect to the shaft, so that a large vibration and noise are produced.